The central objective of this research is to determine the cytoarchitecture, projection patterns, and functional significance of acetylcholinesterase (AChE)-containing neurons in the substantia nigra. We have developed a pharmaco-histochemical method for AChE that, compared to previous regimens for the enzyme, permits improved visualization of AChE-containing neuronal somata and processes in the central nervous system. Using our AChE protocol in conjunction with other histological and histochemical methods and with lesioning and chemical infusion technics, we have presented evidence that AChE is contained within dopamine neurons in pars compacta of the substantia nigra, possibly to inactivate acetylcholine released from cholinergic afferents to the nigra. I propose continuing and extending this work by (1) ascertaining the topography of AChE-containing neurons in the substantia nigra according to our pharmaco-histochemical protocol, (2) determining the efferent connections of these AChE neurons, as well as afferents to them, and (3) elucidating the implications for behavior of the substantia nigra, particularly the role in behavior of cholinergic-monoaminergic interactions. It is hoped that this research will shed light on brain mechanisms for cholinergic-monoaminergic interactions in Parkinsonism and Huntington's disease.